1. Field of the Invention
This invention generally concerns an apparatus and method for hanging a casing liner in a borehole, and more specifically concerns a balanced load apparatus and method for hanging and rotating liners in a well bore during cementing operations.
2. The Prior Art
In oil and gas well operations after drilling the borehole, well pipe is run into the borehole and cemented in place. Basically, well pipe cementing operations are conducted as follows. Liner pipe is suspended in the borehole from hangers on existing well casing and extends to a point in close proximity with the borehole bottom. At the lower end of the liner pipe there is a cement shoe which typically has several orifices. The cement is introduced at the wellhead and passes through the liner to the cement shoe where it passes through the orifices into the borehole. As more cement is introduced, it is forced through the orifices in the shoe and up the well bore on the outside of the liner. As the cement is forced back up the well bore it displaces drilling mud. Generally, it is desirable during cementing operations to rotate and/or reciprocate the casing liner to ensure an even and thorough application of cement and proper displacement of the drilling mud. Rotation during cementing operations is practiced industry wide based upon tests showing 85% mud displacement with rotation and only 30% mud displacement without rotation. Currently, in order to rotate during cementing, the liner must remain attached to the drill string, which is suspended from the drilling rig at the surface, until the liner hanger is set and the drill pipe is released and pulled out of the well. Once set the casing liner is suspended from the well casing by using a liner hanger and it is not permitted to rest on the bottom of the borehole. Suspension of the casing liner in this manner prevents fouling of the cement shoe orifice and prevents the liner from buckling or deforming under its own weight.
Successful utilization of the above described procedure requires precise performance of the drill string releasing mechanism and the setting of the liner hangers. If either the releasing mechanism or the hanger fails to perform properly the result can be very costly. To illustrate this, consider the results of failure of the liner hanger to set properly. In this situation, cementing operations are conducted while rotating the casing liner with the attached drill pipe. Once the cementing is complete, the hanger is set. However, if it fails to set properly it cannot be removed and reset since the cement is already in place. The same is true if the releasing mechanism fails to disengage the drill string, that is, the cement will prevent later removal of the drill string. The results are costly loss of well equipment or even junking of the well. To avoid this problem, most operators set the hanger and release the drill string from it prior to pumping the cement downhole. However, doing so prevents reciprocation or rotation during cementing operations since the drill string must be attached to the casing liner to rotate it. Hence, using this method would result in poor cementing operations.
Thereafter, to alleviate the problem of incapability of rotation once a hanger was set, new hangers were designed to include bearings. In this type of structure, the hanger was set with the casing liner suspended from it and all of the weight was supported by the rotational bearing. Thus, it was possible to cement and rotate after the hanger was set. Although this was a better system, there were still serious problems that resulted in failure. In very deep wells in which the weight of the suspended liner pipe was great, the bearings were subjected to a great deal of stress and would wear out rapidly. Further, these bearings were an open type and therefore were exposed to an extremely erosive well bore environment, i.e., drilling mud, sands, etc., which would also cause excessive wear. Further, with bearings that were open to the erosive well bore environment, it was necessary to lubricate and cool the bearing during its operation. A further problem encountered with the bearings on the prior art hangers was experienced when the well was drilled off center, that is, drilled at an angle from the vertical. Under these circumstances, heavy side pressure on the ball bearing raceway would cause it to crack and break.
Other techniques utilized to solve the weight suspension problems, such as utilizing telescoping liners with intermediate support at each juncture, have not always been successful. Further, it is not possible to rotate the telescoping liners and therefore the cementing process is not as efficient as it could be. Thus, there is a critical need for a liner hanger which is capable of supporting heavy loads and yet is rotational, while allowing release of the setting tool and drill pipe prior to commencing cementing operations.
The present invention provides a unique series of cooperating liner hangers which distribute the load on the bearings proportionately among the individual hangers. Each hanger assembly is located a selected distance away from the others, as required by stress analysis for each individual job, and each is set into position separately within the well casing. A unique sealed bearing arrangement on each of the hangers provides rotational capability. The sealed bearings are automatically pressure balanced to euqalize internal bearing pressure and external ambient pressure at any given well depth. Since the bearings are sealed they are protected from the erosive well environment. Further, each bearing is sandwiched between soft metal layers which, in effect, cushion the bearing raceway and thus help to prevent breaking when a heavy side pressure is exerted on the raceway. Hence, the present invention achieves the need for a rotational, distributed load, liner hanger having a balanced pressure sealed bearing.